The demographics suggest that we face a devastating increase in the prevalence of AD, reinforcing the immediate need for basic and translational neuroscience to develop safe and efficacious ET and HT regimens for the brain. Of those affected with AD, 68% are female and 32% are male (Brookmeyer et al., 1998 Am J Public Health 88:13372). Because women have a longer life expectancy than men, the absolute number of women with AD exceeds that of men. However, a double danger exists for women. Results of a meta-analysis of seven sex-specific studies concluded that women are 1.5 times more likely to develop AD than age-matched men (Gao et al., 1998 Arch Gen Psychiatry 55:809), which was supported by the Cache County analysis that showed a clear female gender increase in the incidence of AD (Zandi et al., 2002 JAMA 288:21239).
At the turn of the new millennium in the United States, there were nearly 42 million women over the age of 50 years and, of these, more than 31 million women were over the age of 55 years (North American Menopause Society, 2004). Worldwide, there are currently more than 470 million women aged 50 years or older, and 30% of those are projected to live into their 80s (North American Menopause Society, 2004). These women can anticipate spending one-third to one-half of their lifetime in the menopausal state. Reports on prevalence of AD vary, but of the 18 million American women in their mid to late 70s, as many as 5 million may suffer from AD, and this figure increases dramatically at older ages (Brookmeyer et al., 1998). The projected exponential increase in the prevalence of AD, along with the anticipated impact on families and society, highlights the imperative for developing strategies to prevent or delay the onset of AD sooner rather than later.
The profound disparities between the largely positive basic science findings of gonadal steroidal action in brain and the adverse outcomes of recent estrogen or hormone therapy (“ET/HT”) clinical trials in women who are either aged postmenopausal or postmenopausal with Alzheimer's disease (AD), has led to an intense reassessment of gonadal hormone action and the model systems used in basic and clinical science. One key factor that could contribute to the negative results of the Women's Health Initiative Memory Study (“WHIMS”) trial was the advanced age, more than ten years following menopause, at which ET/HT was initiated in women. Data from both basic science analyses and clinical studies indicate a “healthy cell bias” of estrogen action in the neurons/brains, suggesting that ET/HT acts as an effective preventative therapeutic strategy for age-related cognitive decline and neurodegenerative disorders, such as Alzheimer's disease (“AD”), while it is not an effective treatment strategy.
The current widely prescribed ET, conjugated equine estrogens (“CEE”), is a highly complex ET with over 200 different components. Whether CEE provides the optimal therapeutic efficacy has been questioned. Another key issue challenging HT is the optimal composition. The progestin and its timing of administration in conjunction with ET, remains to be determined. Moreover, while ET/HT has long been used in postmenopausal women to delay or reverse some of the problems associated with menopause, epidemiological and clinical studies have uncovered potential long-term risks related to this therapy. The recently revealed risks associated with ET/HT have greatly increased interest in the development of estrogen alternatives that promote beneficial effects of estrogen in brain, bone and the cardiovascular system, while not eliciting deleterious effects in other organs, particularly in breast and uterine tissues.
Two nuclear receptors for estrogen (ERs), ERα and ERβ, have been identified. In the central nervous system, both ERα and ERβ are expressed in the hippocampus and cortex of rodent and human brains. Previous studies have demonstrated that both ERα and ERβ can equivalently promote neuronal survival by activating estrogen mechanisms of action in rat hippocampal neurons. Increasing evidence indicates that ERβ is a key requirement for activation of mechanisms that underlie estrogen-inducible neuronal morphological plasticity, brain development, and cognition. ERα, on the other hand, is more predominant in mediating the sexual characteristics of estrogen effects in the reproductive organs such as breast and uterus. Taken together, these data establish a potential therapeutic application for ERβ as a pharmacological target to promote memory function and neuronal defense mechanisms against age-related neurodegeneration such as Alzheimer's disease (AD), while avoiding activating untoward estrogenic proliferative effects in the breast and uterus, although this might be at the cost of lower efficacy due to the lack of activation of ERβ in the brain. Other potential therapeutic advantages associated with ERβ include regulation of estrogen vasculoprotective action and development of interventions targeting diseases such as depression, colon cancer, prostate cancer, obesity, leukemia, and infertility. However, a potential disadvantage of an ERβ-selective ligand is the lack of activation of ERα in bone, as ERα has been demonstrated to mediate estrogen regulation of bone density.
Although there is still controversy regarding the differential roles of two estrogen receptor (“ER”) subtypes, ERα and/or ERβ, in mediating estrogen actions in the brain and/or neurons, it has been widely demonstrated that ERβ plays a key role in regulating brain development, neurogenesis and estrogen-induced improved neuronal plasticity and survival. In addition, as compared with ERα, ERβ is less effective in mediating the sexual characteristics of estrogen action in reproductive tissues, avoiding activating untoward estrogenic proliferative effects in the breast and uterus. Therefore, ERβ represents a potentially safer therapeutic target for promoting memory function and neuroprotection. However, this safety may be at the cost of lower efficacy, due to the lack of activation of Ma in the brain. Other potential advantages for ERβ-target therapeutics arise from its regulation of estrogen's cardioprotective effects. ERβ-selective ligands may also provide effective therapeutics for preventing or treating inflammation, depression, anxiety, colon cancer, prostate cancer, obesity, leukaemia, and infertility.
In searching for an effective ERβ-selective estrogen alternative replacement therapy for promoting neurological function and preventing age-related neurodegeneration, such as AD, in postmenopausal women, it is of particular interest to identify and develop naturally occurring molecules or analogues that potentially have a less toxic profile for long-term administration. It is known that several plant-derived estrogenic molecules (referred to as “phytoestrogens”) bind to ERα and to ERβ subtypes, and some of these molecules possess moderate binding selectivity for ERβ and exert estrogenic effects in multiple tissues.
The therapeutic efficacy of phytoestrogens in the brain remains controversial. On the one hand, when administered singly, phytoestrogens appeared to be moderately neuroprotective. On the other hand, a recent clinical trial revealed that a soy protein supplement that contains a mixture of phytoestrogens did not show improved cognitive function in postmenopausal women, when treatment was initiated at the age of 60 years or older. The clinical trial of phytoestrogens reported that a soy protein supplement containing a complex formulation of isoflavones did not improve cognitive function in postmenopausal women when treated at the age of 60 years or older, Kreijkamp-Kaspers, et al. JAMA 2004, 292, 65-74, also indicating that when started 10 or more years following menopause in postmenopausal women when age-related neuronal reorganization has taken place, ET/HT has no benefit on neural function. Age and hormonal “history” may be important factors that were responsible for these negative results, as was the case for the WHIMS trials.
Another issue that can substantially impact the efficacy of a mixture of phytoestrogens action in the brain is the formulation of phytoestrogens, since when administered alone, a number of phytoestrogens were protective to neurons from neurodegenerative insults. Zhao, et al. Exp. Biol. Med. 2002, 227, 509-519. Soy extracts or soy protein supplements generally contain multiple phytoestrogenic molecules, some of which may be ERα-selective agonists, while others may be ERβ-selective agonists, and others may be ineffective in activating either ERα or ERβ but may function as inhibitors of ER binding of those ERα and/or ERβ phytoestrogenic agonists. The ineffectiveness of a complex formulation of phytoestrogens in promoting beneficial effects of estrogen in brain, such as a soy-derived preparation, may also arise from antagonizing actions among the different phytoestrogens, in addition to the possible ER antagonism, likely from the activation of both ERα and ERR in the same context. Co-administration of an ERα-selective agonist and an ERβ-selective agonist is less effective than treatment with either agonist alone in various neuroprotective measurements.
ERα and ERβ have a yin/yang relationship in many contexts where one receptor may antagonize the actions of the other. Weihua, et al. FEES Lett. 2003, 546, 17-24; Gustafsson, J. A. Trends Pharmacol. Sci. 2003, 24, 479-485. Studies confirmed this observation, showing that coadministration of ERα-selective agonist PPT and ERβ-selective agonist DPN was less efficacious than either PPT or DPN alone in protecting hippocampal neurons against excitotoxic insults. Based on this analysis, a presumption can be made that the ineffectiveness of administering a mixture of phytoestrogens (i.e. a soy protein supplement) may partly come from the antagonizing actions among different phytoestrogens, which may be ERα selective or ERβ selective. These findings indicate that although both ERα and ERβ contribute to estrogen promotion of neuronal survival, simultaneous activation of both ER subtypes, ERα and ERβ, in the same context may diminish the efficacy. In addition, the different ratio and distinct function of homodimer and heterodimer induced by co-administration of an ERα-selective agonist and an ERβ-selective agonist may also account for the reduced efficacy exerted by the combination of both agonists.
Development of an ERβ-selective phytoestrogen formulation could maximize the therapeutic benefits associated with activation of ERβ in the brain while minimizing the adverse effects associated with the activation of ERα in reproductive tissues. Moreover, selective targeting of ERβ potentially reduces antagonistic actions that may occur in a complex soy-derived preparation. This naturally occurring ideal formulation would have tremendous therapeutic value in promoting neurological function and preventing AD in a population at risk for losing neurological capacity and losing memory function, i.e., postmenopausal women. To date, no such phytoestrogen formulation exists. Thus, there is a need to discover and develop a novel select phytoestrogen formulation, generally, and particularly, a formulation that functions in the brain.
It is therefore an object of the present invention to provide an ERβ-selective phytoestrogen formulation maximizing the therapeutic benefits associated with activation of ERβ in the brain while minimizing the adverse effects associated with the activation of ERα in reproductive tissues.
It is a further object of the invention to provide such a composition wherein the active ingredients are isolated from natural substances.